1. Field of Invention
The present invention relates to a vibration isolating apparatus which is used in an engine mount or the like of a vehicle and which damps and absorbs vibrations from a vibration generating portion.
2. Description of the Related Art
In automobile engines, a vibration isolating apparatus serving as an engine mount is disposed between the engine and the vehicle body. The vibration isolating apparatus prevents vibration of the engine from being transferred to the vehicle body.
As this type of vibration isolating apparatus, a vibration isolating apparatus having three restricting passages is known. The first restricting passage absorbs shake vibration (e.g., vibration of a frequency of less than 15 Hz). The second restricting passage has lower transit resistance than the first restricting passage, and absorbs idle vibration (e.g., vibration of a frequency of 15 to 30 Hz). The third restricting passage has lower transit resistance than the second restricting passage, and absorbs shaking sounds (e.g., vibration of 60 to 200 Hz) which are of a higher frequency range than idle vibration.
The vibration isolating apparatus is provided with an opening and closing member controlled by a control means. The opening and closing member is controlled by the controlling means, in accordance with the frequency of the vibration generated by the vibration generating portion, so as to be set in a first state, in which the second restricting passage is closed and the third restricting passage is open, and a second state, in which the third restricting passage is closed and the second restricting passage is open, and a third state, in which both the second restricting passage and the third restricting passage are closed.
In the above-described vibration isolating apparatus, when idle vibration of a low frequency range is generated by the vibration generating portion, the opening and closing member is set in the second state. In this state, liquid passes through the second restricting passage and moves reciprocally between a main liquid chamber and an sub-liquid chamber. Accordingly, vibrations generated by the vibration generating portion are absorbed by liquid-column resonance of the liquid in the second restricting passage.
When vibration of a high frequency range is generated by the vibration generating portion, the opening and closing member is set in the first state. In this state, the liquid passes through the third restricting passage and moves reciprocally between the main liquid chamber and the sub-liquid chamber. Accordingly, vibrations generated by the vibration generating portion are absorbed by liquid-column resonance of the liquid in the third restricting passage.
When shake vibration is generated by the vibration generating portion, the opening and closing member is set in the third state. In this state, the liquid passes through the first restricting passage and moves reciprocally between the main liquid chamber and the sub-liquid chamber. Accordingly, vibrations generated by the vibration generating portion are absorbed by resistance when the liquid passes through the first restricting passage and by liquid-column resonance.
When, for example, alternate generation of shaking sounds and shake vibration in the vehicle engine is repeated, each time the vehicle speed and the engine speed vary, the control means must determine, based on the vehicle speed, the engine speed or the like, whether the vibration generated by the vibration generating portion is high frequency vibration or low frequency vibration, and the control means must control the opening and closing member to be set in the first state or the third state. A drawback arises in that control of the opening and closing member is complicated.
Further, when shaking sound and shake vibration are generated repeatedly in short cycles, it is difficult to reliably drive the opening and closing member synchronously with the cycles.